EarthWorks Geospatial Catalog

UA Census Block Groups, 1990 - Virginia

Description

The polygons represent U.S. Census Block Groups for the state based on the ground condition of January 1, 1990. Block Groups (BGs) are clusters of blocks within the same census tract having the same first digit of their 4-digit census block number. For example, block group 3 (BG 3) within a census tract includes all blocks numbered from 3000 to 3999.
Census 2000 BGs generally contain between 600 and 3,000 people, with an optimum size of 1,500 people. Most BGs were delineated by local participants in the U.S. Census Bureau's Participant Statistical Areas Program. The U.S. Census Bureau delineated BGs only where a local or tribal government declined to participate or where the U.S. Census Bureau could not identify a potential local participant.
A BG usually covers a contiguous area. Each census tract contains at least one BG and BGs are uniquely numbered within census tract. Within the standard census geographic hierarchy BGs never cross county or census tract boundaries, but may cross the boundaries of county subdivisions, places, urbanized areas, voting districts, congressional districts, and American Indian/Alaska Native areas/Hawaiian home lands. Under the Census 2000 American Indian/Alaska Native area/Hawaiian homeland census geographic hierarchy, census tracts and BGs are defined within American Indian entities and can cross state and county boundaries. These are commonly referred to as Tribal BGs.
BGs have a valid range of 0 through 9. BGs beginning with a 0 generally are in coastal and Great Lakes water and territorial seas. Rather than extending a census tract boundary into the Great Lakes or out to the three mile territorial sea limit, the U.S. Census Bureau delineated some census tract boundaries along the shoreline or just offshore. The U.S. Census Bureau assigned a default census tract number of 0000 and BG of 0 to the offshore areas not included in regularly numbered census tract areas.
In decennial census data tabulations, a block group may be split to present data for every unique combination of county subdivision, place, voting district, congressional district, American Indian area/Alaska Native area/ Hawaiian home land shown in the data tabulation products.